Tremolo harmonica

This gives a unique wavering or warbling sound created by the two reeds being not exactly in tune with each other and difference in their subsequent waveforms acting against one another.

This effect is fairly common amongst Western free-reed instruments and is found in accordions, harmoniums and reed organs under various names (celeste, vox jubilante, etc.

Most tremolo harmonicas are built upon what is termed the "Wiener system", named after the city of Vienna (Wien in Austria) where they first emerged.

Unlike the standard ten-hole harmonicas (built on the "Richter system", e.g. Hohner's Marine Band) the blow and draw reeds do not share a common chamber, but are separated off from one another.

In Asia, the fourths and the sixths are added back in, in order to play the melody; however, it is still unlike the scale tuning mentioned below, since the octaves are not repeated throughout the layout.

In the West, the tremolo harmonica is usually encountered in traditional folk music, being found throughout Europe and South America in this role.

Players often use several different harmonicas at a time, holding them one atop the other, in order to play notes and chords not available on any single instrument.